Slurries prepared for pipeline transportation generally must be maintained in an agitated state during temporary storage. Otherwise, the particles in the slurries will settle out and be difficult to put back into suspension in the slurry carrier. Ideally, the means for agitating the slurry should produce and maintain a uniform particle concentration and particle size consist through the volume of slurry contained in the tank. The concentration and consist should at least be stable and predictable at the point of discharge from the storage tank; otherwise, upset conditions could occur in the pipeline transporting the slurry.
The current practice in active slurry storage makes use of a storage tank in the shape of a right circular cylinder. A propeller like paddle is located near the tank bottom and is rotated about a vertical axis in a horizontal plane, the axis of rotation generally being coincident with the axis of the cylindrical tank. In large tanks, two paddles on the same shaft are needed to develop sufficient agitation force. Vertical baffles are usually attached to the cylinder walls to provide for vertical motion of the slurry rather than horizontal motion.
The flow induced in the agitated slurry in a conventional apparatus is toroidal with the axis of the toroid being coincident with the axis of the storage tank. A large amount of power is required to establish and maintain this motion not only because the solids and the slurry carrier are circulated but also because the particles are lifted by fluid entrainment. Thus, a high flow velocity is developed to entrain the particles and carry them to a location near the top of the slurry in the tank. Having reached the upper ends of their paths of travel, the particles are immediately carried downwardly by the same entrainment flow to repeat the cycle. This is an undesirable feature because of the power requirements and a need exists for improvements in the handling of coal and other particle slurries in active slurry storage situations to avoid or minimize this problem.
Representative disclosures in the field of slurry storage tanks include U.S. Pat. Nos. 4,332,484 and 4,367,048.